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THE TRANSPORTATION OF GASES
179
The pressure coefficient corresponding to the observed pressure rise (pz - pd, is CP = A2ytta2, (for notation see p. 176). The theoretical power coefficient is Ct = 0.0257lCe(7p. The characteristic curve for Ct should be computed and drawn from readings from the smooth curve of Cp against Ce. The sAa/i power coefficient is C8 = shaft horsepower/wa3. The rotation loss coefficient is Cr = 0.0737 X lQ~QDadm, where dTO is the average density of the gas between pi and p^ Ib. per cubic foot. By adding the values of Cr to the values of C», a characteristic curve (practically a straight line) of fluid input plus rotation loss is obtained, and this curve will in a correctly designed compressor nearly touch the shaft power characteristic curve at the value of Q/ua corresponding to the rated load of the compressor.
The ratio of Ct to d for any value of Ce gives the hydraulic efficiency eh for that particular load. (For Ce = 0, the general formula eh = 5,955 CP must be used). Similarly, ratios of Ct to Cs give values of et, the shaft efficiency. The efficiency curves thus obtained will of course be smoother and more reliable than if the efficiencies were computed directly from the individual observations of pressure and power.
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C, Load Coeff.  » Q/N|                j
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FIG. 35.—Characteristic curves of centrifugal compressors.
Uses of Characteristic Curves.—Besides affording smooth curves of hydraulic and shaft efficiencies, a set of characteristic curves as shown in Fig. 35 enables one readily to draw reliable pressure and power curves against quantity for any given wheel speed or revolutions per minute. In that case, the load-coefficient scale may be replaced by a quantity scale, while the readings of the Cp and the C, curves will give the data for the corresponding pressures and powers. (In Fig. 35 the wheel speed has, in all the coefficients, been replaced by the revolutions per minute). The system of characteristic curves is found to hold true for various sizes of centrifugal compressors, centrifugal blowers, and centrifugal pumps, giving in each case consistent curves regardless of the actual speeds, pressures and powers. Only in ventilating-